The working element can be formed by, for example, a folding press that has at least one upper tool and one lower tool, which cooperate to bend and bevel workpieces comprising pieces of sheet metal, in particular. For this purpose, the upper tool is guided toward the lower tool in a pressing movement, thereby deforming a piece of sheet metal, which is disposed between the two tools, along a fold line. Folding presses of this type are potentially hazardous to the operator, because the workpiece must usually be guided manually during the pressing movement. Consequently, the operator's hands and fingers are near the upper and lower tools, or near the squeezing and shearing points of the folding press or the workpiece, which puts the operator at risk for serious injury.
To make these detection regions safer, it is standard practice to use protective devices that disable the working element if the operator has inadvertently intruded into the detector region.
These protective devices can take the form of two-handed switches that protect the operator's hands by requiring the actuation of two levers to initiate and maintain a potentially dangerous movement in a folding press. The operator must therefore keep both hands on the levers, which safeguards him against injury during press operation.
A drawback of this device, however, is that other persons in the detection region are unprotected. A further drawback is that the operator cannot manually guide the workpiece during machining, which can be problematic. The operator therefore often chooses to disable the protective device and guide the workpiece manually in the detection region, without protection.
Safety light curtains are also used as protective devices. DE 39 39 191 C3, for example, discloses such a device. In this case, the safety light curtains comprise numerous pairs of transmitters and receivers that respectively form a light barrier.
The light beams emitted by the transmitters and aimed at the respectively associated receivers cover a planar, two-dimensional monitoring region. It is possible to detect that a hand is approaching a detection region, whereupon the working element is disabled.
A safety light curtain of this nature is typically disposed vertically, and serves to bar access to a folding press. In the machining of larger pieces, it is advantageous to arrange the safety light curtain horizontally to secure the region. A disadvantage of this arrangement is that a plurality of safety light curtains is required for completely securing the detection region of this folding press, especially with respect to preventing access and securing the region. This requirement not only raises costs to an undesirable level, but is also associated with complicated assembly, often with only insufficient space available for the devices.
DE 196 19 688 A1 relates to a method for video monitoring of operator areas, such as of a press, in which a video camera optically detects the operator area to be monitored, and generates an electronically-processable image of the open operator space, which the camera supplies to an image processor, where it is stored.
Prior to each startup of the press, the video camera records an image of the operator area and compares the generated image to the image stored in the image processor. The press can only be started up if the new image is identical to the stored image.
A disadvantage of this method is that any deviation of a current image from the reference image disables the press, even if the deviation would not pose a threat to the respective operator. This system results in a low availability of the press, which unnecessarily limits the operating times of the press.